Effects of heating on the mechanical and chemical properties of human dentin.

We had previously discovered that the flexural and tensile strengths of human dentin were 2-2.4 times greater after being heated to 140°C, and deduced that the generation of higher-density structures and therefore dehydration probably promoted the increased strength. Our test hypotheses were that intertubular dentin, which constitutes a major part of organic components, was selectively affected by heating, and such changes could happen without critical damages to the basic structure of dentin type I collagen.Micro-mechanical changes of human dentin by heating at 140°C were investigated by nano-indentation. Chemical changes in dentin collagen after heating were also investigated by X-ray diffraction study, a microscopic Fourier transform infrared (micro-FTIR) and a laser Raman spectroscopic analyses, and a cross-linking analysis by high-performance liquid chromatography.The results of nano-indentation showed that the micro-hardness of intertubular dentin increased after heating at 140°C to 1.8 times more than unheated dentin; on the other hand, peritubular dentin was unchanged. Results of X-ray diffraction showed that the lateral packing of collagen molecules shrank from 13.6±0.3 to 10.6±0.1Å after heating, but the shrinkage reversed to the original after rehydration for seven days. After heating, no substantial chemical changes in the collagen molecules were detected in tests by micro-FTIR or Raman analyses, or by cross-linking analysis.These results suggest that intertubular dentin, which contains most of the type I collagen, was selectively affected by heating at 140°C without critical damage to its collagen.